The invention concerns a method to control stops of an intermittently operating electric motor, such as a driving motor for a pump or a mixer, the stop being dependent of the current consumption of the motor.
When using pumps it is important for several reasons to avoid unnecessary dry running of the pump because it causes increased wear, additional current consumption, etc.
Different types of stopping methods and devices are known. The most common being to sense the pump head which means that the pump is stopped at a certain minimum level. A disadvantage with this solution is that the pump head level indicator is often sensitive to mechanical and chemical influences which presents a risk of significant errors.
One way to detect a dry running pump without any level indicator is to analyze the motor current and sense its decrease which occurs when the pump starts to run dry. This can be obtained by comparing the value of actual current level with a fixed level or by detecting a change in the current.
Turning the motor off at a certain current level can of course be accomplished by connection of a current sensitive relay. However, such a device is only a good solution during stable conditions and when the pumping fluid is relatively clean. At changing operating conditions such as when the pump is moving or where abrasive fluids are being pumped, the level of current consumption changes relatively quick causing the relay to release at the wrong time. The improper triggering of the relay can also be a source of error.
Stopping of the motor depending on the change of the current has also certain disadvantages. At pump start up, the stop function must be by-passed for a certain period (blank time), otherwise a stop would take place immediately. But, this also means that a stop would not occur if the pump runs dry at start up because a current decrease had already taken place before the end of the blank time period.
The disadvantages mentioned about are eliminated by the invention hereinafter described.